Soil microbes is a general term for all microorganisms in soil, including bacteria, archaea, fungi, viruses, protozoa and microalgae¹. Soil serves as a natural medium for microbial growth and reproduction, which encompasses diverse and abundant soil microbial resources². In soil ecosystems, soil organisms rely on and restrict each other and interact with environmental factors. Although the relationship between the soil microorganisms and other organisms is complex, there are six major relationships: intergrowth, symbiosis, parasitism, antagonism, predation and competition¹. Soil health refers to a dynamic process whereby soil, having a good granular structure and functional state, can provide continuous and stable productivity, to maintain the ecological balance and environmental quality. Healthy soil is the key to sustaining healthy plants and ensuring food safety³. It is also an important buffer for environmental change and environmental pollution.

    Soil microorganisms play an important role in maintaining soil health through many life activities and nutrient cycling, thus ensuring the soil ecosystem functions and nutrients are utilized sustainably⁴. There is a close relationship between microbial community structure, diversity, soil-borne diseases and soil status⁵. Mycorrhizal symbiosis is a common feature of most terrestrial plants, and mycorrhizal association exists in almost all ecosystems, including desert, tropical forest and farmland⁶. Approximately 86% of the vascular plants have symbiosis with mycorrhizal fungi, and more plants may form interactions with the non-mycorrhizal fungi⁶. Orchidaceae is one of the largest and diverse groups in the plant community, the origin of which is ancient, and is still undergoing rapid differentiation and species formation. Orchidaceae is a typical mycorrhizal plant family, and occupies a unique and very important position in the evolution of biological systems^7,8 (Figure 1).            

   The seed of Orchidaceae is very small, without endosperm, and it is difficult to germinate under natural conditions without symbiotic fungi⁹. Therefore, in the early development stage of seed to the seedling, which is non-photosynthetic, the Orchidaceae plants entirely rely on the mycorrhizal fungi. Orchids are divided into three types: photoautotrophic, mixed and fully mycoheterotrophic - based on the carbon sources¹⁰ (Figure 2). Current Orchidaceae research is mainly focused on the relationship between the mycorrhizal fungi and its host. Our research team have used ITS1 high-throughput sequencing and rhizosphere microbiome of orchids. On basis of the influence of sampling method, sampling time and regional geography on experimental design, the root-endophytic and rhizosphere microbial diversity and gene function of orchids were elucidated and the relationships were revealed. Furthermore, we proposed the “orchid--endophytic—rhizosphere microbiome” three-way carbon source linkage mode of nutrient interaction. Based on co-evolutionary analysis, we proposed the “Orchidaceae-rhizobiome”as a co-evolutionary hypothesis, with the root endophytic microbiome as an organic entity, co-evolving with its host orchids.

    We have many new technologies such as more effective sequencing techniques and geographic information systems (GIS) to uncover deeper biological knowledge. Using these stepping stones, many orchid genomes are analyzed and published, providing a basis for studying the nutritional relationship between orchids and symbiotic fungi. In addition, studies on mycorrhizal fungi and non-mycorrhizal fungi are of great significance to the protection of rare and endangered orchids.

References

1. Somani, L. L., Bhandari, S. C., Bhandari, S. C., & Somani, L. L. (1994). Soil organisms. Ecology & Biology of Soil Organisms.

2. Hanne N. Rasmussen. (2002). Recent developments in the study of orchid mycorrhiza. Plant & Soil, 244(1), 149-163.

3. Hibma, J. . (2013). Soil health. Countryside & Small Stock Journal.

4. Schnitzer, S. A. , Klironomos, J. N. , Hillerislambers, J. , Kinkel, L. L. , Reich, P. B. , & Xiao, K. , et al. (2011). Soil microbes drive the classic plant diversity-productivity pattern. Ecology, 92(2), 296-303.

5. Pankhurst, C. E. , Doube, B. M. , & Gupta, V. V. S. R. . (1997). Biological Indicators of Soil Health. Biological Indicators of Soil health..

6. Bonfante, P. , & Genre, A. .(2010). Mechanisms underlying beneficial plant-fungus interactions in mycorrhizal symbiosis. Nature Communications, 1(4), 48.

7. Héctor Herrera, Inmaculada García-Romera, Meneses, C. , Pereira, G. , & César Arriagada. (2019). Orchid mycorrhizal interactions on the pacific side of the Andes from Chile. a review. Journal of Soil Science and Plant Nutrition, 19(1), 187-202.

8. Zelmer, C. D. , Cuthbertson, L. , & Currah, R. S. . (1996). Fungi associated with terrestrial orchid mycorrhizas, seeds and protocorms. Mycoscience, 37(4), 439-448.

9. Miura, C. , Yamaguchi, K. , Miyahara, R. , Yamamoto, T. , & Kaminaka, H. . (2018). The mycoheterotrophic symbiosis between orchids and mycorrhizal fungi possesses major components shared with mutualistic plant-mycorrhizal symbioses. Molecular Plant-Microbe Interactions, 31(10), 181-190.

10. Ai, Y., Xie T. X., et al. 2019. Community structure and biological function of the root symbiotic fungi of Arundina graminifolia. Mycosystema, 38(10):1631-1642.

Figure 1. Cymbidium ensifolium in wild condition. (photo: Zhong-Jian Liu)

Figure 2. Ghost orchid, Epipogium roseum in the natural habitat (photo: Zhong-Jian Liu)

This post was led by Zhong-Jian Liu and co-written with Xue-Die Liu, at the Key Laboratory of National Forestry and Grassland Administration for Orchid Conservation and Utilization at College of Forestry, Fujian Agriculture and Forestry University.